1. Field of the Invention
The present invention relates to an X-ray computed tomography (CT) scanner that reconstructs a diagnostic image of a subject from X-ray detection data collected by irradiating the subject with an X-ray and a data processing method of the X-ray CT scanner, and more particularly to an X-ray CT scanner including a plurality of bulbs and a data processing method of the X-ray CT scanner.
2. Description of the Related Art
An X-ray CT scanner is an apparatus that irradiates a subject with an X-ray from a bulb of an X-ray tube and performs image reconstruction processing with respect to X-ray detection data collected by an X-ray detector to reconstruct a diagnostic image of the subject. As one of diagnostic image reconstruction techniques using this X-ray CT scanner, there is half reconstruction. According to the half reconstruction, projection data of an angle less than 360 degrees (generally, projection data of 180 degrees+a fan angle) is used and one image is thereby generated to reconstruct a diagnostic image, whereas data corresponding to a direction of 360 degrees is detected with respect to a subject to reconstruct a diagnostic image in regular image reconstruction processing.
This half reconstruction technology can reconstruct an image from projection data in a small angle range and can obtain an image with a high time resolution, and hence it is often used to obtain an image of a part having a local movement, e.g., the heart. In particular, an image with less influence of a movement can be obtained by obtaining an image of the heart based on half reconstruction of X-ray detection data of an angle less than 360 degrees collected based on electrocardiographic synchronization.
FIG. 1 is a conceptual view for explaining a method of collecting half reconstruction data based on electrocardiographic synchronization using a conventional X-ray CT scanner.
As shown in FIG. 1, in a beat period of an ECT (Electrocardiogram) signal, a subject is irradiated with an X-ray from a bulb #1, and half data Dh is collected by an X-ray detector. Further, the half data Dh depicted in FIG. 1 is data corresponding to approximately 210 to 240 degrees, e.g., approximately 240 degrees.
However, when the number of beats per minute is 120, since a time tb of one heartbeat is 0.5 s, a comparable time resolution is required when collecting data in one heartbeat. However, collecting sufficient half data is usually difficult in one heartbeat in many cases, and a technique of dividing data into segments over a plurality of heartbeats to collect half data has been proposed.
FIG. 2 is a conceptual view for explaining a technique of dividing half reconstruction data into segments based on eletrocardiographic synchronization to collect the data by using a conventional X-ray CT scanner.
As indicated by a bulb trajectory P in FIG. 2, a technique of helically moving the bulb around a subject and collecting one segment S1 of half data in a first cardiac beam B1 and two segments S2 and 3 of the half data in second and third heartbeats B2 and B3 has been proposed. Furthermore, the half data of approximately 240 degrees in a reconstruction plane Y can be obtained based on data collection in the three heartbeats.
On the other hand, a multi-bulb X-ray CT scanner has been designed as an attempt to improve a time resolution. The multi-bulb X-ray CT scanner is an apparatus having a structure where a plurality of bulbs are provided in the X-ray CT scanner and an X-ray detector arranged to face each bulb detects an X-ray applied to a subject from each bulb.
In this multi-bulb X-ray CT scanner, a three-bulb X-ray CT scanner including three bulbs has been designed in particular. In case of the three-bulb X-ray CT scanner, an apparatus in which pairs of three bulbs and three detectors are uniformly arranged at 120 degrees has been designed (see, e.g., JP-A 168616-1993 [KOKAI] and JP-A 2001-346791 [KOKAI]). When collecting data corresponding to 360 degrees, using this three-bulb X-ray CT scanner enables collecting the data corresponding to 360 degrees with a rotating angle of 120 degrees. Therefore, as compared with the X-ray CT scanner having one bulb, it can be expected that data can be ideally collected in a time reduced to ⅓.
FIG. 3 is a conceptual view for explaining a technique when collecting data by using the conventionally designed three-bulb X-ray CT scanner.
As shown in a state St01 in FIG. 3, when three bulbs #1, #2, and #3 are equally arranged at angle intervals of 120 degrees, a state St02 is obtained by rotating each of the bulbs #1, #2, and #3 at 120 degrees, thereby collecting data corresponding to 360 degrees.
Therefore, when a rotating speed of each bulb is 0.3 s/rotation, a time required to collect the data corresponding to 360 degrees is 120/360×0.3=0.1 s, which is ⅓ of that in an example where one bulb is used.
FIG. 4 is a conceptual view showing data collected by the conventionally designed three-bulb X-ray CT scanner.
In FIG. 4, an ordinate represents a data collection range expressed as an irradiation angle of an X-ray with respect to a subject, and an abscissa represents a channel (Ch) used by each X-ray detector. As shown in FIG. 4, each X-ray detector facing each of three bulbs #1, #2, and #3 collects data corresponding to 120 degrees in each different region. Furthermore, the number of channels of a detection element provided in each X-ray detector is the same, and X-rays are detected in all the channels.
As a result, as shown in FIG. 4, each X-ray detector collects data D#1′, D#2′, and D#3′ each corresponding to 120 degrees in accordance with the number of channels, the number of pieces of data being the same in the respective detectors. That is, the three-bulb X-ray CT scanner collects data corresponding to 360 degrees that is in proportion to the number of channels.
Moreover, it is considered that the multi-bulb X-ray CT scanner is effective for high-speed scanning in terms of such a time resolution.
In imaging using the X-ray CT scanner, as explained above, an improvement in a time resolution is an important problem. Thus, utilization of the multi-bulb X-ray scanner is expected for data collection requiring a time resolution. However, an irradiation amount of X-rays is increased when the number of bulbs is increased, and hence an exposure amount of a subject grows. Additionally, when the number of bulbs is increased, a size of each X-ray detector is increased, thus pushing up a manufacturing cost.
In general, a time resolution is required when an imaging field of view (FOV) is mainly narrow and an image of a local region, e.g., the heart is imaged in particular. Thus, when a fan angle of an X-ray tube and a size of each X-ray detector are reduced, imaging in a wide region with a relatively low time resolution becomes difficult. That is, when reducing the size of the X-ray detector to assure a high time resolution with a smaller irradiation amount of X-rays, assuring a sufficiently wide FOV is difficult.
Further, when the multi-bulb X-ray CT scanner in which bulbs having the same size are evenly arranged is used, 360-degree rotation according to the number of the bulbs does not have to be carried out, and a time resolution can be improved as compared with an example where the X-ray CT scanner having a single bulb is used to perform full reconstruction. However, in order to collect data that is continuous in terms of angles, full reconstruction must be carried out, and using the half reconstruction technology is difficult. Therefore, when capturing an image of a local region, a further improvement in a time resolution is demanded.